The present invention relates to a print wire driving apparatus, and more particularly to an apparatus for improving power supply efficiency in driving print wire.
Wire dot print heads and print wire drivers are well known in the art. A conventional wire dot print head, shown in FIG. 12 includes a core 12 having urging coils 11 wound thereabout. A plurality of wires 14 are supported on springs 13 for selectively extending the portions of wires 14 beyond the nose of wire dot print head 10. Urging coils 11 are energized to attract springs 13 which in turn impart kinetic energy to wires 14. This causes wires 14 to extend through wire dot print head 10 to strike an ink ribbon forming characters or figures in the form of a dot matrix. Wire dot print head 10 may include eight to sixty-four wires 14 depending on the use to which the printer is to be applied.
Reference is now made to FIG. 9 in which a conventional apparatus for driving wire dot print head 10, generally indicated as 20, is provided. Apparatus 20 includes a power source 2, a drive circuit 5 coupled to power source 2 and a drive signal generator 6 also coupled to drive circuit 5. Power source 2 includes a pair of input terminals 1 and 1' which are coupled to a voltage source. A capacitor C.sub.0 and a stabilizing circuit 3 are in parallel and connected across input terminals 1 and 1'. A diode PD is connected to input terminal 1' and to a first terminal A. The output of stabilizing circuit 3 is electrically connected to terminal A. A smoothing capacitor C.sub.1 is electrically coupled to input terminal 1' and a first terminal B. A smoothing inductor PL is electrically coupled between terminal A and terminal B.
Power source 2 emits a stabilized voltage through feedback of the voltage across terminal B to stabilizing circuit 3 along an input 4. Diode PD is designed so that stabilizing circuit 3 allows current to follow in a gated manner in accordance with the electromagnetic energy accumulated in smoothing inductor PL to charge smoothing capacitor C.sub.1.
Drive signal generator 6 includes a shift register 9 which sequentially store input data corresponding to the characters of figures to be printed under the control of the input from a shift clock. A latch circuit 8 simultaneously latches the data accumulated in shift register 9 in response to input latch pulses. An enable circuit 7 restricts the time period in which latch circuit 8 provides an output in response to an input enable signal. Driving signal generator 6 supplies data which is adapted for use by print head 10 to drive circuit 5.
Drive circuit 5 includes a plurality of urging coils L.sub.i, transistors TR.sub.i and diodes D.sub.i. For simplicity, a single grouping of these elements is described. An urging coil L.sub.1 is coupled between a first terminal 1 and one end of an N type transistor TR.sub.1 having its gate coupled to enable circuit 7 and its other end coupled to input terminal 1'. The anode and cathode of a diode D.sub.1 are coupled at an end of urging coil L.sub.1 and a zener diode ZD, respectively. Each of the diodes D.sub.1 and D.sub.i are connected to a single zener diode ZD. The number i corresponds to the number of print wires 14 contained within print head 10.
Reference is now made to FIGS. 10a and 10b in which equivalent circuits generally indicated as 100a, 100b of respective prior art apparatus 20 are shown in which it is assumed that a voltage V.sub.1 at terminal B equals 30 V. The inductance of urging coil L.sub.i is set at L=3 mH and has a resistance R.sub.L =20.OMEGA.. The equivalent resistance of transistor TR.sub.i, R.sub.T, is set at 0.5 .OMEGA. and the equivalent voltage V.sub.Z of zener diode ZD is set at 75 volts and has an equivalent resistance R.sub.Z of 0.5 .OMEGA.. Equivalent circuit 100a is the circuit resulting when transistor TR.sub.i is ON and circuit 100b illustrates a case in which transistor TR.sub.i is OFF.
The current i.sub.1 of equivalent circuit 100a can be expressed as follows: ##EQU1## and t is time.
To calculate the energy of the system, the period of time in which transistor TR.sub.i is ON is assumed to be 200 .mu.s. Energy P.sub.IN1 supplied by power source 2 is expressed as follows: ##EQU2##
The energy consumed due to the resistance of equivalent circuit 100a having a total resistance of 20.5 .OMEGA. is expressed as follows: ##EQU3##
The energy P.sub.L accumulated within urging coil L.sub.i is: ##EQU4##
The foregoing equations may be checked based on the conservation of energy so that the power supplied by the power source should equal the energy consumed by the resistor and the energy accumulated by the coil. Accordingly, it follows that: ##EQU5##
Reference is now made to FIG. 10b and equivalent circuit 100b which represents apparatus 20 when transistor TR.sub.i is OFF. If the current i.sub.2 is determined based upon the following underlying assumptions that: ##EQU6##
If time .tau. when i.sub.2 =0 is determined, then .tau.=5.88.times.10.sup.-5 sec.
Reference is now made FIG. 11 wherein the currents i.sub.1, i.sub.2 derived above are graphically displayed. A maximum cycle period of 500 .mu.s is illustrated to provide a time cushion between ensuing operation of the urging coil. The time cushion is required since vibrations that are not directly related to striking occur after print wire 14 has struck an ink ribbon until print wire 14 returns to its original stationary position. The energy produced and utilized during the time period 0 to .tau., corresponding to the production of current i.sub.2 is calculated by first calculating energy P.sub.IN2 supplied by power source 2. ##EQU7##
The energy P.sub.R2 consumed by the overall resistance of equivalent circuit 100b having a value of 20.5 .OMEGA. is: ##EQU8##
The energy P.sub.ZD consumed which is attributable to the equivalent voltage of the zener diode ZD is expressed as follows: ##EQU9##
The energy consumed which is attributable to the 0.5 .OMEGA. resistance of zener diode ZD is expressed as follows: ##EQU10##
Therefore, total energy consumed by zener diode ZD is expressed as follows: EQU P.sub.TZD =2.25+0.01=2.26 mJ.
The total energy P.sub.IN supplied by power source may be expressed as follows: ##EQU11##
Accordingly, the energy consumed by zener diode ZD which is equal to 2.26 J accounts for as much as 46% of the energy supplied by power source 2. Zener diode ZD is required by apparatus 20 to quickly cut off current i.sub.2 to operate print wires 14 at a high speed. The higher the voltage of zener diode ZD, the more quickly current i.sub.2 can be cut off.
In the actual prior art device, assuming that the number of dot wires is 24 and that the repetition frequency is 2 kHz, the power P supplied by power source 2 is expressed as follows: ##EQU12##
The actual power consumption P.sub.Z of zener diode ZD is expressed as follows: ##EQU13##
As can be seen, roughly 46% of the power utilized by the system is consumed by the zener diode.
Most of the energy supplied by the power source in apparatus 20 is consumed by the resistance of the urging coils and zener diode resulting in a large conversion of energy into heat. As the number of print wires and operating speed increases, design of a print head which satisfactorily reduces energy lost through heat becomes more difficult. Furthermore, as the number of print wires increase a large capacity power source which can instantly supply the power must be utilized since the coils are simultaneously energized.
Accordingly, it is desirable to provide an apparatus for driving a wire dot print head which overcomes the shortcomings of the prior art by efficiently utilizing the energy supplied by the power source.